Monotube Flash Boiler Design

[TimTaylor]

The current pressure washer pump has a single piston which moves back and forth within a cylinder. Pressurized water is collected from both cylinder ends, effectively giving this pump two cylinders. The swashplate axial design would have 9 cylinders, with each cylinder pushing out slightly less than 1/4 the volume per cylinder, as compared to the current 2 cylinder pump, which should significantly reduce the "power" of each pulse, yes?

My analogy is this: each pressure pulse from the pump is like a hammer blow. A 2 cylinder pump is like having 2 very large hammers generating shock waves, while a 9 piston pump is like having 9 much smaller hammers generating lots of much smaller shock waves.

I'm also approaching this from my electronics background where I know that rectifying 400 Hz and smoothing the output to a clean DC level, is much, much easier than starting with 60 Hz. In other words, an accumulator on the output of a 9 piston pump should work much better than an accumulator on a 2 piston pump, yes - no ??

From what I've read on the web, 7 and 9 piston pumps are significantly smoother than the more common 3 piston pumps. Yes - no??

Assuming equal displacement, the individual pulses will be smaller, but the overall energy will be about the same or even slightly greater due to combined efficiency losses. What will change is the pulse frequency - if you use a 9 piston pump, the pulse frequency for any given speed will be 4.5X that of a 2 piston pump. You'll still need some type of pulsation damper. Unlike your projected several months to design/build a new pump, you could build a damper with mostly off the shelf components in a few days.

 

[Richard Hed]

Pressure washer pumps are positive displacement pumps and the pressure pulses you are seeing are a normal characteristic of their operation. The frequency of the vibration in CPM will be the number of pistons multiplied by the run speed of the pump motor, so it will vary with motor speed. Those vibrations can be destructive, especially if they excite a structural resonant frequency.

You'll never completely get rid of them, but as Steamchick suggested, a pulsation dampener downstream of the pump would definitely help level them out. Typically it's a larger diameter vertical pipe section with either some trapped air or a bladder in the top 1/3, which provides a compressible volume for the pulses of non-compressible water to work against and help average them out.

Actually, that is exactly how an engine muffler works too.

 

[The Ignoble Troll]

Assuming equal displacement, the individual pulses will be smaller, but the overall energy will be about the same or even slightly greater due to combined efficiency losses. What will change is the pulse frequency - if you use a 9 piston pump, the pulse frequency for any given speed will be 4.5X that of a 2 piston pump. You'll still need some type of pulsation damper. Unlike your projected several months to design/build a new pump, you could build a damper with mostly off the shelf components in a few days.

The energy in totality would be the same, but going with your assumptions on displacement - the wave form would be drawn out vs sharp waves. Otherwise the flow would be increased many fold.

So at the same flow and the more the pistons, the slower the piston cycling which means the more gentle the pulse.


Even 3 single acting pistons on a crank shaft is enough to produce a smooth enough flow for commercial high pressure boilers ie. A steam pressure washer, and not fatigue the boiler pipe.

 

[TimTaylor]

The energy in totality would be the same, but going with your assumptions on displacement - the wave form would be drawn out vs sharp waves. Otherwise the flow would be increased many fold.

So at the same flow and the more the pistons, the slower the piston cycling which means the more gentle the pulse.


Even 3 single acting pistons on a crank shaft is enough to produce a smooth enough flow for commercial high pressure boilers ie. A steam pressure washer, and not fatigue the boiler pipe.

Actually high pressure boiler systems (defined as operating at > 80 bar) almost universally use multi-stage barrel or ring pumps, either in a single pump or main+booster configuration. The pumps also don't dead head into the boiler, but operate in conjunction with a bypass feedwater regulator.

Properly designed (smaller pistons vs motor speed), the flow would not change at all. Water is an incompressible fluid, and each piston stroke will deliver a specific volume of liquid and generate a pressure pulse. Unlike other types such as centrifugal or vane pumps, they are not affected by things like cavitation. The vibration excitation frequency of those pulses (CPM) is equal to the motor speed multiplied times the number of cylinders/pistons - if you prefer HZ, divide CPM by 60. It's a simple law of physics.

All mechanical systems have resonant frequencies. If an excitation frequency coincides with a resonant frequency it can be very destructive and cause component or system failure. A pulsation dampener averages out the peaks and reduces the excitation energy amplitude.

 

[TimTaylor]

Actually, that is exactly how an engine muffler works too.

Sort of - engine exhaust is a compressible fluid, while water is not, so the characteristics are different.

 

[The Ignoble Troll]

Actually high pressure boiler systems (defined as operating at > 80 bar) almost universally use multi-stage barrel or ring pumps, either in a single pump or main+booster configuration. The pumps also don't dead head into the boiler, but operate in conjunction with a bypass feedwater regulator.

Properly designed (smaller pistons vs motor speed), the flow would not change at all. Water is an incompressible fluid, and each piston stroke will deliver a specific volume of liquid and generate a pressure pulse. Unlike other types such as centrifugal or vane pumps, they are not affected by things like cavitation. The vibration excitation frequency of those pulses (CPM) is equal to the motor speed multiplied times the number of cylinders/pistons - if you prefer HZ, divide CPM by 60. It's a simple law of physics.

All mechanical systems have resonant frequencies. If an excitation frequency coincides with a resonant frequency it can be very destructive and cause component or system failure. A pulsation dampener averages out the peaks and reduces the excitation energy amplitude.

Commercial pressure gas washers run from a pump to a boiler operating 100+ bar use triplex pumps. That's a three piston pump feedinging a heater or boiler with an rpm typically in the 3600-3400 range. The technology is ubiquitous.

 

[TimTaylor]

Commercial pressure gas washers run from a pump to a boiler operating 100+ bar use triplex pumps. That's a three piston pump feedinging a heater or boiler with an rpm typically in the 3600-3400 range. The technology is ubiquitous.

Apples & oranges -

In both hot water and wet steam pressure washers the hot water vessel and related piping upstream of the pump on the low pressure side - the only things downstream of the pump are the flexible high pressure hose and the washing gun.

In boilers, the pressure vessel and piping are downstream of the feedwater pump on the high pressure side. The applications are totally different and not comparable.

 

[The Ignoble Troll]

Apples & oranges -

In both hot water and wet steam pressure washers the hot water vessel and related piping upstream of the pump on the low pressure side - the only things downstream of the pump are the flexible high pressure hose and the washing gun.

In boilers, the pressure vessel and piping are downstream of the feedwater pump on the high pressure side. The applications are totally different and not comparable.

You have that backwards. Only a handful of companies make pumps that handle hot water. The majority build pumps that suffer fatal cavitation, or valve deformation, if their inflow water is heated much above a very hot day or if the water is well aerated. These pumps are used to pressurize a heated coil/boiler. You can buy 285 bar units with pressurized boilers off the shelf in north America.

Related units with electrically driven pumps are used in self serve car washes.

Far cheaper then the ceramic pumps it takes to pump hot water.


My back ground is in mid sized systems. Both maintenance and design.

Piston pumps are also roundly used in hydraulics as the pinnacle of efficient smooth pumping.

 

[The Ignoble Troll]

Do you still have the hose from the pressure washer? If so, does it have a heat rating on the layline?

If so, is it high enough to try and use it as a dampener between the pump and the boiler?

 

[Toymaker]

Do you still have the hose from the pressure washer? If so, does it have a heat rating on the layline?

If so, is it high enough to try and use it as a dampener between the pump and the boiler?

The marking on the hose shows: 18 MPA 60 C. As the project progresses to having a hot well tank, I'll need to replace the pressure washer tubing.

I do like your idea though,...a length of plastic or rubber tube should work to absorb at least some of the energy from those strong pulses

 

[Toymaker]

Question for the pump experts:

Below is a graph of what I believe the Pressure out over time (or shaft rotation) of a 9 piston swash plate pump would look like. The GREEN lines show the pressure & flow rate of the 4 pistons which are actively supplying pressure and flow at any given time in the pumps rotation. Because there are 4 pistons pushing fluid out at all times, pressure at the output should never drop below the "Average" line.

Am I right or wrong ??

 

[The Ignoble Troll]

Question for the pump experts:

Below is a graph of what I believe the Pressure out over time (or shaft rotation) of a 9 piston swash plate pump would look like. The GREEN lines show the pressure & flow rate of the 4 pistons which are actively supplying pressure and flow at any given time in the pumps rotation. Because there are 4 pistons pushing fluid out at all times, pressure at the output should never drop below the "Average" line.

Am I right or wrong ??

View attachment 156716
View attachment 156717

Looks different from other graphs I've seen of multi piston pump pressure outputs.

Does not look wrong to me, just different.

I will try and find you a paper with a equivalent graph.

 

[The Ignoble Troll]

Here is one

https://www.researchgate.net/figure...f-9-pistons-of-the-piston-pump_fig2_348661459

...but yours makes more sense to me...

 

[The Ignoble Troll]

https://www.researchgate.net/figure...s-for-partial-stroke-disabling_fig2_311374755

This one makes more sense to me. The piston is under full load as soon as it is actuated positively.

 

[Toymaker]

https://www.researchgate.net/figure...s-for-partial-stroke-disabling_fig2_311374755

This one makes more sense to me. The piston is under full load as soon as it is actuated positively.

Thank you for your inputs; after re-thinking a bit, I can now see how pressure is, as you stated, nearly instantaneous, building rapidly as soon as the swash plate begins to push the piston and the valve plate rotates to open that piston's output. So, my graph shows only flow rate, not pressure.

I don't see how either of the graphs you linked are accurate, as both show a slow decrease in flow rate, slowly dropping all the way down to zero. The contour of the swash plate will cause each piston to move up slowly at first, perhaps through the first 15 degrees of rotation, then the piston will move up faster in the mid-section before slowing again during the last 15 degrees of rotation. Just before the piston reaches TDC (top dead center), the valve plate's rotation closes off flow from that piston nearly instantaneously, as my graph shows.

Still, the important data (at least for me) shown in all the graphs is the fact that with multi-piston pumps, the output pressure never drops to zero. Where-as with a two piston pump, such as the cordless pressure washer pump I'm currently using, both flow and pressure drop to zero as each of the two pistons reach their respective TDC, thereby causing the rather violent pulsing vibration I'm seeing.

 

[Nerd1000]

Actually, that is exactly how an engine muffler works too.

Engine mufflers are complicated. They work at least partly by creating destructive interference in the sound waves, which is why they often have multiple different sized chambers inside, to affect different frequencies.

How bad is it to have pulsations in the boiler feed? Would think that the compressibility of the steam would soak it up mostly.

 

[Toymaker]

Engine mufflers are complicated. They work at least partly by creating destructive interference in the sound waves, which is why they often have multiple different sized chambers inside, to affect different frequencies.

How bad is it to have pulsations in the boiler feed? Would think that the compressibility of the steam would soak it up mostly.

In a mono-tube (or water tube) boiler, at pressures of 4oo to 500 psi, the majority length of boiler tube will be incompressible liquid water,...only a relatively short length of boiler tube will be filled with steam. And since the steam will be at high pressure, I'm not very confident it will act as much of a cushion to absorb the pump's pulses. Perhaps I'm overly concerned about the strong pulses generated by this 2 piston feed pump; the previous 3 piston pump generated much, much less vibration & pulsing.

 

[The Ignoble Troll]

Thank you for your inputs; after re-thinking a bit, I can now see how pressure is, as you stated, nearly instantaneous, building rapidly as soon as the swash plate begins to push the piston and the valve plate rotates to open that piston's output. So, my graph shows only flow rate, not pressure.

I don't see how either of the graphs you linked are accurate, as both show a slow decrease in flow rate, slowly dropping all the way down to zero. The contour of the swash plate will cause each piston to move up slowly at first, perhaps through the first 15 degrees of rotation, then the piston will move up faster in the mid-section before slowing again during the last 15 degrees of rotation. Just before the piston reaches TDC (top dead center), the valve plate's rotation closes off flow from that piston nearly instantaneously, as my graph shows.

Still, the important data (at least for me) shown in all the graphs is the fact that with multi-piston pumps, the output pressure never drops to zero. Where-as with a two piston pump, such as the cordless pressure washer pump I'm currently using, both flow and pressure drop to zero as each of the two pistons reach their respective TDC, thereby causing the rather violent pulsing vibration I'm seeing.

That makes sense to me, especially the difference between the axial and double acting pump.

The reason both triplex and axial pumps have 3 pistons is the pressure wave overlap. If those double acting pumps of yours are holding up to the heat, what about driving 3 off of jack shafts from a common motor. Then you'd have 6 pulses that you could overlap?

I tried to find a chart showing actual piston velocities in a fixed swash plate system, but all I could find was papers on variable flow swash plate units, where the piston travel is not fixed.

Seeing the actual actuation behavior of the pistons on a graph would have been nice.


In terms of resonance between the pump output and coils, I personally think it would be obvious. The license plate on my bike has a couple points where it resonates with the little bike engine and it causes a very audible racket. your system should act in kind, with resonance... well... resonating.


It seems to me that if you added a couple of accelerometers to your unit, in places they won't get cooked, an arduino should give you the pressure spike induced system vibrations as a simple to graph read out. Any sudden spikes in the vibration would let you know if you hit resonance.

To be clear it wouldn't likely show the pump graph but the overall system "noise" as I assume that the tubes generate lots of vibration as they vaporize the water.

The units are cheap so it wouldn't break the bank, if you are concerned about resonance.


Does that make sense?

Oh boy I can't wait for this move to be over. Then you guys can dog pile my projects


Since, as I recall correct me if I am wrong, you're boiler is intended to operate above the critical point, you might have more cushioning then you think as you will have the water phase changes to absorb the pulses?

Edit: I was staring right at the forest but missed it because a bunch of trees were in the way

check out fig 6

https://tud.qucosa.de/api/qucosa:71101/attachment/ATT-0/

It shows the behavior of a single piston on a fixed plate.

Actually most of the paper is useful because while it is about variable plates, it is comparing them to fixed plates.

Hope this helps.

 

[Nerd1000]

In a mono-tube (or water tube) boiler, at pressures of 4oo to 500 psi, the majority length of boiler tube will be incompressible liquid water,...only a relatively short length of boiler tube will be filled with steam. And since the steam will be at high pressure, I'm not very confident it will act as much of a cushion to absorb the pump's pulses. Perhaps I'm overly concerned about the strong pulses generated by this 2 piston feed pump; the previous 3 piston pump generated much, much less vibration & pulsing.

I just had the thought that the volume of the steam pipes going to your turbine might be big enough to give you some cushioning from the steam itself acting as a spring.

 

[Toymaker]

I just had the thought that the volume of the steam pipes going to your turbine might be big enough to give you some cushioning from the steam itself acting as a spring.

Hmmm, that might just work,...but I'm at least several weeks away from being able to test that theory.
So, as they used to say on old radio shows, "stay tuned"